Improve time stamp reader from the hdf5 file

[jrutyna]

Currently, the hdf.py module (see line 30) is using the "Time Date Stamp" table from the hdf5 file. This is okay to use unless the HEC-RAS output is set to sub-second intervals then the "Time Date Stamp (ms)" table should be used. Need to investigate if this has any adverse impacts elsewhere in the repository before making this change.